A huge sell order left corn lagging its Chicago crop peers, which again faced the headwind of a strong dollar.
There was some confusion surrounding the sale, which occurred about 15 minutes into the live trading day.
"There are various stories going around," Vic Lespinasse, at GrainAnalyst.com, said.
"Most say a very large order was entered electronically for between 3,000-8,000 contracts to sell March corn, supposedly on a sell stop.
"Some say this was only supposed to be an order for 300-800 contracts and by mistake another 0 was added."
At broker Country Futures, Darrell Holaday said: "It has been a mixed, but wild morning in the grains spurred by a 6,000-contract sell stop in the corn market that pushed the corn market $0.18 [a bushel] lower."
Indeed, the March contract fell to $4.00 ½ a bushel at one point before recovering most of the lost ground to stand down 3.25 cents at $4.13 ¾ a bushel in late deals.
If investors needed another excuse to be wary, the dollar provided some by making headway against most major currencies, hitting a three-month high against the yen and touching $1.4274 against the euro. A stronger dollar makes US exports such as crops less competitive.
And corn had the additional setback of a 2m-tonne jump, to 15.8m tonnes, in the Buenos Aires Grain Exchange's estimate for Argentina's corn crop.
The US Department of Agriculture estimates production at 14.0m tonnes.
For soybeans, the news was mixed. Taiwan passed on a tender for a cargo of US or Brazilian soybeans, citing high price, but is expected to retender on Thursday.
Meanwhile, the USDA reported the sale of 348,000 tonnes of American soybeans to China for 2009-10 delivery.
"Brazilian cargoes are offered at roughly the same price in February, but China continues to secure US soy with crush margins still in the green and freight costs well above a year ago," US Commodities, the Iowa-based broker, said.
January soybeans stood 2.5 cents lower at $10.35 ½ a bushel, with the better-traded March lot down 3.25 cents at $10.43 ¾ a bushel.
Wheat did best, up 2 cents to $5.43 for March delivery, a move put down to continued speculation about next month's index and rebalancing.
The annual shake-up favours commodity laggards (such as grains for 2009) by raising their weightings back to benchmark, a process which means sales of better-performing contracts.
The latest news from Benson Quinn Commodities was that the DJ-UBS commodities index would "base its reweighting for its basket of commodities on prices as of the close on January 7".
The broker added: "The reweighting is expected to bring buying into the agriculture markets. The rebalancing will begin on January 8 and run for five days."
Among softs, sugar continued to garner the headlines, hitting a fresh record in London of $708.00 a tonne for the near-term (March) contract.
Prices were supported by news of plans by Pakistan for a tender for 150,000 tonnes of white sugar, the type that London trades, with Iraq approving an order to import 250,000 to cover it needs for the next eight months.
The contract closed at $705.00 a tonne, up $4.80 on the day, with New York raw sugar for March rising 0.7% to 26.92 cents a pound.
Another notable mover was orange juice, which lost 2.7% for March to close at 136.9 cents a pound.
Profit-taking was blamed amid some disappointment - among bulls, if not growers – that no freeze is in the forecast for Florida, America's biggest citrus state.